Maps in iOS 6 will require A5 processor for 3D “flyover,” navigation

iPhone 3GS in particular will get a reduced feature set in iOS 6.

iOS 6 promises to bring lots of new features to iPhone and iPad users—more than 200, according to Apple's count. And the software will run on several generations of past hardware, including the original iPad and the iPhone 3GS. But many of the new features won't work on every device; for instance, the impressive 3D Flyover view and turn-by-turn navigation in Maps will only work with A5-powered devices. That means those features will only work on the iPhone 4S, iPad 2, and iPad 3, but not the iPhone 4, iPhone 3GS. And iPhone 3GS users will miss out on even more from iOS 6, including Siri, FaceTime over 3G, and VIP Mail lists.

Apple has posted an extensive preview of iOS on its website, detailing many of the features highlighted during its keynote presentation at its annual Worldwide Developers Conference on Monday. Tucked in at the bottom of the page, among the usual fine print, is the warning that "[n]ot all features are available on all devices."

This is nothing new; successive generations of iPhone hardware have faster processors, more capable GPUs, and sometimes other functions like hardware-based encryption. For instance, Siri has so far been limited to the iPhone 4S, though Apple will bring its AI-based "virtual assistant" to the latest-generation iPad when iOS 6 ships this fall.

The iPhone 4 won't be gaining Siri either, but the iPhone 3GS will miss out on that and many more of iOS 6's prominent features. That includes using FaceTime over cellular connections—there's no front-facing camera—but it also includes software-based features like Shared Photo Streams, VIP list and VIP and Flagged mailboxes in Mail, Offline Reading List.

Apple still sells the iPhone 3GS as new (in addition to the iPhone 4 and iPhone 4S), so it's only right that the company continue to support them with iOS updates. It's not clear why some of the listed features were cut from the 3GS, as many don't appear to be particularly processor-intensive. It's also likely that other features may turn up missing once iOS 6 ships later this year.